It is known that acrylamide (or methacrylamide) can be prepared from acrylonitrile (or methacrylonitrile) in the presence of a free metallic copper catalyst such as Raney copper or reduced copper (see US Patent Application, Ser. No. 56,967, filed on July 21, 1970, owned by the assignee of the patent application). When executing such a method industrially, a continuous manufacturing process is carried out at a reaction temperature in the range of 60.degree.-200.degree. C and in the presence of a catalyst in the form of fixed or fluid bed, the amount of the catalyst to be used is in the range of 1-1,000 g per each 1 mol of acrylonitrile (or methacrylonitrile) used as a raw material. When a reduced copper is employed as a catalyst, a fixed bed is commonly used in consideration of the grain size of the catalyst. On the other hand, when Raney copper or a reduced copper of a relatively larger grain size is employed, a fluid bed is commonly used.
In general, when a continuous reaction is to be carried out using of any one of the catalysts as mentioned above, a preselected amount of raw materials and catalyst which are mixed together into a slurry state are introduced into the reactor and, at the same time, a portion of the slurry is taken out of the reactor to be supplied into a liquid cyclone or thickener, the greater part of the catalyst is separated, and then the residual catalyst in the reaction liquid is filtered off.
In such a continuous manufacturing process of acrylamide, the reaction is performed in the presence of catalyst having a relatively high concentration as mentioned above. This occurs, when such an ordinary method is employed, a number of problems as described below:
The acrylamide synthesizing reaction is characterized in that the catalyst is used in a large quantity compared with the reaction liquid and additionally the catalyst is used in a porous or spongy state so that, handled by a pump during after-treatment for the reaction product, the catalyst may readily be crushed into pieces of smaller mesh sizes. Especially when a centrifugal slurry pump is utilized, the catalyst will be crushed and increasingly finer particles will be produced as the time passes, which will reduce the separating efficiency of the thickener and therefore increase the amount of the catalyst to be filtered.
The point of great importance is that the change in the grain size of the catalyst will lead to the variation in the surface area of the catalyst so that, even if the other conditions are maintained constant, the amount of reaction per each unit amount of catalyst will be varied and, therefore, a conversion of acrylonitrile cannot be maintained constant.